I have seen recently that it is said that keywords in bold, italic and underline add to your SEO - how? This is when you use it in context links, for example: The cheap designer shoes are available in red and black, but the cheap designer shoes are new and still got tags. Get the point? So why is this useful to SEO?
Italic, bolded, and heading text does impact SEO to some degree. When Google or another search engine crawls your site it looks at all of the HTML for bolded, italic, underlined text. Additionally, it will look at links and their anchor, alt tags, title tags, and other matadata. Italic and Bolded text helps your visitors identify key items on your site so the web bot do the same thing - identify the important text on your site.
I would highlight the keyword once and to give more room for more relevant keywords to be highlighted. And yes, highlight, underline, bold have some value in SEO to show the importance of your keywords since meta keywords are completely ignored in SEO ranking in Google. There are over 200 factors that measures our ranking in SERP. This could be one of it; that leave 199+ more factors to be considered.
I have one example for this... why there are speed breaker on the road, when speed breaker comes then you slow there and take a time as this bold or italic is also a speed breaker for crawler and where if found bold or italic its take time on this crawling and it is beneficial for us so keep your keyword bold and italic.
What a nice example for keyword stuffing, unfortunately the original question was about bolding, underlining etc your keywords. Yes, as already stated, Google will take this as a hint (and more) for determining what is the main topic / are the main topics of the text. SY
They helps because search engine treats bold and italic text as more important than the regular text. Just like search engine see <h1> as the most important text on the paragraph.